Ferroin is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula e(''o''-phen)₃O₄, where ''o''-phen is an abbreviation for 1,10-

phenanthroline 1,10-Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. The 1,10 refer to the location of the nitrogen atoms that replace CH's in the hydrocarbon called phenanthrene. Abbreviated ...

, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.

Redox indicator

This coordination compound is used as an indicator in

analytical chemistry Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

. The active ingredient is the e(''o''-phen)₃sup>2+ ion, which is a chromophore that can be oxidized to the ferric derivative e(''o''-phen)₃sup>3+. The potential for this redox change is +1.06 volts in 1 M H₂SO₄. It is a popular redox indicator for visualizing oscillatory

Belousov–Zhabotinsky reaction A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in ...

s. Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in

cerimetry Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric chemical analysis developed by Ion Atanasiu. It is a redox titration in which an iron(II)– 1,10-phenanthroline complex ( ferroin) color change indicates t ...

.{{cite book, title=Handbook on the Physics and Chemistry of Rare Earths, url=https://books.google.com/books?id=_Ro3Fqtz4xgC&pg=PA289, date= 2006, publisher=Elsevier, isbn=978-0-08-046672-9, pages=289– Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has transition potential of +1.25 volts. It is more stable than ferroin, but in sulfuric acid with Ce⁴⁺ ion it requires significant excess of the titrant. It is however useful for titration in

perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous s ...

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

solution, where cerium redox potential is higher. The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by adjusting the position and number of methyl groups on the phenanthroline core.

Preparation

Ferroin sulfate may be prepared by combining

to ferrous sulfate in water. :3 phen + Fe²⁺ → e(phen)₃sup>2+ The iron is low spin and octahedral with D₃ symmetry. The intense color of this ferrous complex arises from a metal-to-ligand charge-transfer transition.

Reaction kinetics

When sulfuric acid (H₂SO₄) is added to a solution of e(phen)₃sup>2+, it causes the iron complex to decompose as described by the following reaction: : e(phen)₃sup>2+_(aq) + 3 H₃O⁺_(l) → Fe²⁺_(aq) + 3 phenH⁺_(aq) + 3 H₂O_(l) Thanks to the high

molar absorptivity Molar may refer to: *Molar (tooth), a kind of tooth found in mammals *Molar (grape), another name for the Spanish wine grape Listan Negro *Molar (unit), a unit of concentration equal to 1 mole per litre *Molar mass * Molar volume *El Molar, Tarrago ...

of ferroin (a measure of how much the molecule interacts with light), this decomposition can be observed measuring the solution absorbance over time. The rate of decomposition is first order in e(phen)₃sup>2+, and zeroth order in 3 H₃O⁺. That means the rate law is: :Decrease in ferroin over Time = k erroin where k is the reaction constant, and erroinis the concentration of ferroin. ₃O⁺does not show up in the rate law. The concentration of ferroin over time decreases with exponential decay.

References

Iron complexes Phenanthrolines